1. Field of the Invention
The present invention relates to a method for detecting a substance in a liquid using a surface acoustic wave element (SAW element) and a sensor for detecting a substance in a liquid and, more particularly, to a method for detecting a substance in a liquid that uses a sensor for detecting a substance in a liquid provided including a plurality of sensing SAW elements and a plurality of reference SAW elements, and the sensor for detecting a substance in a liquid.
2. Description of the Related Art
In the related art, in order to detect an antigen, an antibody, a protein, or other substance in a liquid, a sensor for detecting the substance in the liquid that includes a surface acoustic wave element (SAW element) is used. WO2006/027893A1 describes an example of a sensor for detecting a substance in a liquid of this type.
FIG. 6A is a plan view that illustrates a sensor for detecting a substance in a liquid described in WO2006/027893A1. FIG. 6B is a front cross-sectional view that shows the major portion thereof.
The sensor for detecting a substance in a liquid 101 includes a base substrate 102. Recesses 102c and 102d are provided on the upper surface 102a of the base substrate 102. A sensing SAW element 104 and a reference SAW element 105 are respectively provided in the recesses 102c and 102d. A resin layer 103 is laminated on the base substrate 102 so as to be arranged at a distance from one end 102b. The resin layer 103 includes through-holes 103b and 103c that face the recesses 102c and 102d, respectively. The sensing SAW element 104 includes a piezoelectric substrate, an IDT electrode provided on the piezoelectric substrate, and a reaction film arranged so as to cover the IDT electrode. The reaction film is made of a material that reacts with a detection target substance, as a sample, in a liquid to bond with the detection target substance. The reference SAW element 105 includes an IDT electrode provided on a piezoelectric substrate. No reaction film is provided for the reference SAW element 105.
In use, at least portions including the through-holes 103b and 103c are submerged in a liquid, and in the sensing SAW element 104, the reaction film reacts with the detection target substance to bond with the detection target substance. Thus, in the sensing SAW element 104, a mass applied to a portion at which the IDT electrode is provided increases due to the bonding of the detection target substance. In contrast, in the reference SAW element 105, no reaction film that reacts with the detection target substance is provided, so that an increase in additive mass due to bonding of the detection target substance does not occur.
In the sensor for detecting substance in a liquid 101 described in WO2006/027893A1, a variation in acoustic velocity of a surface acoustic wave due to the additive mass in the sensing SAW element 104 is detected as a variation in electrical signal. In this case, by obtaining a difference between an output from the sensing SAW element and an output from the reference SAW element 105, it is possible detect the detection target substance.
In addition, WO2006/027945A1 describes a similar sensor for detecting substance in a liquid.
In the sensor for detecting substance in a liquid 101 described in WO2006/027893A1, if no detection target substance, such as a protein, that reacts with the reaction film is present, the sensing SAW element and the reference SAW element must have substantially the same behavior, that is, substantially the same frequency characteristic. However, among SAW elements, some have large variations in frequency variation when a liquid is attached thereto. When such SAW devices are used as the sensing SAW element or the reference SAW element, the detection sensitivity of a protein, or other substance, decreases and, in some cases, an erroneous detection result may occur.
That is, it may be determined that a detection target substance is present when no detection target substance is actually present, or it may be determined that no detection target substance is present when a detection target substance is actually present.